Introduction
DNA stands for Deoxyribonucleic acid.DNA replication is actually the process of producing two identical strands of DNA. It occurs in all living organisms including prokaryotes and eukaryotes.
DNA is made up of two complementary strands of various nitrogenous bases i.e. Adenine, Guanine, Thymine, and Cytosine.
Adenine binds with Thymine in a double covalent bond and Guanine binds with Cytosine in a triple covalent bond. These long molecules are coiled around histones and then supercoiled in a compact chromosome.
DNA is semiconservative in nature i.e. one of the DNA strands comes from the parental DNA and the other gets synthesized.
23 pairs of chromosomes are present in every single cell in our body. But the cells are constantly dividing. Each new cell gets a specific amount of genetic material with the help of the process of DNA replication
Steps involved in DNA REPLICATION
Replication starts at a specific sequence called Ori or the origin of replication which is identified before the unwinding of DNA strands.
Adenine and Thymine are bound with each other in double covalent bonds whereas Cytosine and Guanine are bound with each other in triple covalent bonds. Therefore it is easy to break the bond formed by Adenine and Thymine.
The enzyme helicase unwinds the double helix and disrupts the hydrogen bonds between base pairs thus separating DNA into two different strands creating a replication fork.
Now the role of another enzyme called primase comes into contact. The enzyme primase will bind an RNA primer to the parental strand which is 5 to 10 nucleotides long
This enzyme helps to bind another enzyme called DNA POLYMERASE and develop a whole new complementary strand adding nucleotides to the new chain. The enzyme polymerase catalyzes the formation of a phosphodiester bond that joins each new nucleotide.
This process would be different for different Strands as the polymerase copies in the direction 3' to 5' only, but the strands are antiparallel to each other. The direction of replication must be in the opposite direction.
Two types of strands are made during the process of replication:
1. Leading strand:-
In the leading strand, DNA replication moves along the replication fork continuously synthesizing the complementary strand- and requiring only the initial primer.
2. Lagging Brand:-
In a lagging strand, primase binds with the parental DNA in the form of chunks or fragments and then DNA polymerase continues to make complementary strands. These fragments are called Okazaki fragments and they are around 100-200 nucleotides long. Each Okazaki fragment will require its own primer to bind and copy the strand.
Lastly, a separate enzyme ligase is responsible to stick together the Okazaki fragments.
Summary
In the whole process of DNA REPLICATION four types of enzymes are involved:
1. Helicase: This enzyme unwinds the helix and separates the strands.
2. Primase: Helps bind another enzyme called polymerase.
3. Polymerase: It copies each strand
4. Ligase: It serves as a sticking enzyme for the Okazaki fragments.